ISO Metric Threads (M) Reference
Callout meaning, Grade II default practice, and ISO tolerance-class alignment for global acceptance
ISO metric threads are specified by nominal diameter and pitch (e.g., M10 × 1.5). In real procurement, misunderstandings usually come from the “acceptance language”: some drawings only show diameter × pitch, while receiving inspection may expect an ISO tolerance class (such as 6H/6g) and a defined measurement basis—especially when plating is involved. UJEN stock items follow a general-purpose practice by default (commonly referenced as Grade II in some supply chains). If your project is specified to ISO standards for gauging and acceptance, please state the ISO tolerance class and whether gauging is required pre- or post-plating at RFQ stage, so drawings, gages, and inspection criteria stay aligned.
What an ISO Metric Thread Callout Means
ISO metric threads are specified by nominal diameter and pitch, written as: M10 × 1.5. The “M” indicates metric, “10” is the nominal diameter (mm), and “1.5” is the pitch (mm per thread).
Recommended Callout Format for Drawings and RFQs
- Basic: M8 × 1.25
- Left-hand thread (if applicable): add “LH”
- Thread length / engagement: specify where functional
- When specified to ISO acceptance: add ISO tolerance class (example: 6H internal / 6g external) and define measurement basis (pre- or post-plating)
Buyer Summary
If your drawing/RFQ does not define an ISO-based acceptance system, specifying M diameter × pitch is commonly used for general procurement. UJEN stock items are commonly supplied under a general-purpose practice (often referenced as Grade II in some supply chains). When a project is specified to ISO standards for gauging and acceptance, please specify the ISO tolerance class (such as 6H/6g or customer-defined) and whether gauging is required pre- or post-plating. Custom acceptance requirements may result in different process routing, pricing, MOQ, and lead time.
Applicable ISO Standards (Metric Threads)
- ISO 261: ISO metric thread series plan (coarse and fine)
- ISO 68-1: basic profile for ISO metric threads
- ISO 724: basic dimensions for ISO metric threads
- ISO 965-1: tolerance system and tolerance classes (e.g., 6H/6g)
- ISO 1502: gauges and gauging for ISO metric threads
Metric Thread Quick Reference (Common Sizes)
This table is a quick reference for common ISO metric thread pitches. For production and acceptance, please follow the drawing/RFQ callout and the specified tolerance class (if any).
| Thread (Diameter) | Common Coarse Pitch | Common Fine Pitch Options | Typical Notes |
|---|---|---|---|
| M3 | 0.5 | 0.35 | Fine pitch used when adjustment resolution is needed |
| M4 | 0.7 | 0.5 | Common for compact assemblies |
| M5 | 0.8 | 0.5 | Frequently used in adjustable components |
| M6 | 1.0 | 0.75 | Coarse for general use; fine for controlled adjustment |
| M8 | 1.25 | 1.0 | Common for knobs, handles, and clamping features |
| M10 | 1.5 | 1.25, 1.0 | Coarse for general use; fine for precision intent |
| M12 | 1.75 | 1.5, 1.25 | Often used where higher load margin is required |
| M16 | 2.0 | 1.5 | Confirm engagement and assembly clearance constraints |
Pitch Selection: Coarse vs Fine
Coarse pitch is common for general fastening and robust assembly. Fine pitch can improve adjustment resolution and support certain design intents, but may be more sensitive to contamination and requires appropriate design margin. Always state pitch explicitly when interchangeability matters.
Thread Tolerance Notes (ISO Classes vs Grade Language)
ISO metric tolerance classes such as 6H (internal) and 6g (external) are defined by a grade number and a deviation letter under ISO 965. Different combinations may be used depending on assembly feel, interchangeability, and the acceptance system used by the buyer.
In some supply chains, legacy grade language such as I / II / III is used as a general-purpose reference. On UJEN stock items, when no special requirement is specified, Grade II is treated as the default general-purpose practice. When a project is specified to ISO standards for acceptance, specify the ISO tolerance class on the drawing/RFQ (for example 6H/6g or customer-defined) and define the measurement basis for plated parts.
Measurement Basis Note (Pre-Plating vs Post-Plating)
By default, thread gauging is performed pre-plating (before coating). If your requirement is that the GO gauge must pass after plating, this requires a different process route (thread allowance, plating control, and inspection method) and will be treated as a custom requirement. Price, MOQ, and lead time may differ.
UJEN Practice (How We Control Threads)
For stock items, UJEN follows a stable general-purpose practice (commonly referenced as Grade II in some supply chains) with consistent internal inspection flow. If your project requires ISO tolerance-class alignment (e.g., 6H/6g or customer-defined) and a specified measurement basis (pre- or post-plating), please state it on the RFQ/drawing so we can align process routing, gauging method, and quotation basis.
RFQ Checklist for Metric Threads
- Thread type: ISO Metric
- Internal or external thread
- Callout: M diameter × pitch
- UJEN stock items: commonly supplied under a general-purpose practice (often referenced as Grade II in some supply chains)
- If specified to ISO acceptance: state ISO tolerance class (e.g., 6H/6g or customer-defined)
- Plating/coating requirement and measurement basis (pre- vs post-plating)